High voltage photovoltaics integrated with light emitting diode

ABSTRACT

An electrical device that includes a material stack present on a supporting substrate. An LED is present in a first end of the material stack having a first set of bandgap materials. A photovoltaic device is present in a second end of the material stack having a second set of bandgap materials. The first end of the material stack being a light receiving end, wherein a widest bandgap material for the first set of bandgap material is greater than a highest bandgap material for the second set of bandgap materials.

BACKGROUND Technical Field

The present invention generally relates to photovoltaic devices, and more particularly to photovoltaic devices used to power light emitting diodes.

Description of the Related Art

A photovoltaic device is a device that converts the energy of incident photons to electromotive force (e.m.f.). Typical photovoltaic devices include solar cells, which are configured to convert the energy in the electromagnetic radiation from the sun to electric energy. A light-emitting diode (LED) is a two-lead semiconductor light source. It is a p-n junction diode, which emits light when activated. When a suitable voltage is applied to the leads to the LED, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.

SUMMARY

In accordance with one embodiment, an electrical device is described herein that includes a photovoltaic structure in combination with a light emitting diode (LED) structure, wherein the photovoltaic structure provides the power to the LED. In one embodiment, a material stack is provided that includes an LED device at a first end of the material stack and a photovoltaic device at a second end of the material stack. The LED device has a first set of bandgap materials, and the photovoltaic device has a second set of bandgap materials, the first end of the material stack being a light receiving end, wherein a wider bandgap material for the first set of bandgap material is greater than a highest bandgap material for the second set of bandgap materials.

In another embodiment, an electrical device is provided that includes a material stack is having a photovoltaic device at a first end of the material stack and an LED structure at a second end of the material stack. The LED device has a first set of bandgap materials, and the photovoltaic device has a second set of bandgap materials, the first end of the material stack being a light receiving end, wherein a wider bandgap material for the second set of bandgap material is greater than a highest bandgap material for the first set of bandgap materials.

In another aspect, a method of forming an electrical device is described herein that in one embodiment includes providing a material stack including an LED portion and a photovoltaic portion. The LED portion is comprised of semiconductor material layers having a first set of bandgaps. The photovoltaic portion is comprised of semiconductor material layers having a second set of bandgaps. The LED portion of the material stack is at a first end of the material stack and the photovoltaic portion is present at a second end of the material stack, wherein the device having the first or second set of bandgaps having the wider band gap is positioned at the light end receiving end of the material stack. In one embodiment, the method of forming the electrical device may include growing a first junction on a supporting substrate; and forming a second junction on the first junction by molecular beam epitaxial growth. Contact may then be formed to the first junction and the second junction, wherein a first device of the first and the second junction is a photovoltaic device, and a second device of the first and the second junction is a light emitting diode. The semiconductor material layers in the LED or the photovoltaic device that is at a light receiving end of the electrical device have a wider band gap than the LED or the photovoltaic device that is not at the light receiving end of the electrical device.

These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will provide details of preferred embodiments with reference to the following figures wherein:

FIG. 1 is a side cross-sectional view depicting one embodiment of a high voltage photovoltaic cell including gallium nitride (GaN) that is integrated with a light emitting diode (LED), in which the LED device is positioned between the supporting substrate and the photovoltaic device.

FIG. 2 is a side cross-sectional view depicting one embodiment of a high voltage photovoltaic cell including gallium nitride (GaN) that is integrated with a light emitting diode (LED), in which the photovoltaic device is positioned between the supporting substrate and the LED.

FIG. 3 is a side cross-sectional view depicting one embodiment of a high voltage photovoltaic cell including aluminum gallium nitride (AlGaN) that is integrated with a light emitting diode (LED), in which the LED device is positioned between the supporting substrate and the photovoltaic device.

FIG. 4 is a side cross-sectional view depicting one embodiment of a high voltage photovoltaic cell including aluminum gallium nitride (GaN) that is integrated with a light emitting diode (LED), in which the photovoltaic device is positioned between the supporting substrate and the LED.

FIG. 5 is a flow chart describing one embodiment of a method for forming high voltage photovoltaics that are integrated with LEDs, in which the method includes a low hydrogen deposition process.

FIG. 6 is a flow chart of another embodiment of a method for forming high voltage photovoltaics that are integrated with LEDs.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosed herein; however, it is to be understood that the disclosed embodiments are merely illustrative of the claimed structures and methods that may be embodied in various forms. In addition, each of the examples given in connection with the various embodiments are intended to be illustrative, and not restrictive. Further, the figures are not necessarily to scale, some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the methods and structures of the present disclosure.

References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

For purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, and derivatives thereof shall relate to the invention, as it is oriented in the drawing figures. The terms “overlying”, “atop”, “positioned on” or “positioned atop” means that a first element, such as a first structure, is present on a second element, such as a second structure, wherein intervening elements, such as an interface structure, e.g. interface layer, may be present between the first element and the second element. The term “direct contact” means that a first element, such as a first structure, and a second element, such as a second structure, are connected without any intermediary conducting, insulating or semiconductor layers at the interface of the two elements.

In one embodiment, the present disclosure provides photovoltaic cells, i.e., photovoltaic devices, needed for internet of things (IOT) applications that also include light emitting diodes (LEDs). As used herein, a “photovoltaic device” is a device, such as a solar cell, that produces free electrons and/or vacancies, i.e., holes, when exposed to radiation, such as light, and results in the production of an electric current. In some embodiments, a multi-junction photovoltaic device includes a multiple junctions of a semiconductor layer of a p-type conductivity that shares an interface with a semiconductor layer of an n-type conductivity, in which the interface provides an electrical junction. As used herein, the term “LED” denotes a semiconductor material containing structure that emits light when an electrical current is passed through it. In some embodiments, the light-emitting diode (LED) is a two-lead semiconductor light source, which can resemble a pn-junction diode. In this example, when a voltage is applied to the leads to the pn-junction type diode, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.

Physically small, i.e., devices having a small footprint, having high voltage requirements are needed. The length and width dimensions of the monolithically formed devices of high voltage photovoltaics integrated with LEDS that are described herein may be no greater than 150 microns, e.g, may be equal to 100 microns or less.

In some embodiments, the voltage requirements can be met by a multi-junction photovoltaic cells, as depicted in FIG. 1. The voltage requirements for a photovoltaic device of this type may be between 2 volts and 3 volts to drive light emitting diodes (LEDs) for communication applications. The voltage requirements to write and read memory cells may range from 2 volts to 3.5 volts. In battery applications, the photovoltaic devices may need to provide 3.5 volts to charge a battery.

In some embodiments, an integrated LED is needed to send signal to outside the device. Monolithically integrated LEDs with photovoltaic devices can reduce size of an electrical device that includes an LED that is separate from a photovoltaic device or other power source. In some embodiments, the structures and methods disclosed herein provide a high voltage photovoltaic device that is integrated with an LED that may be used for energy harvesting and communication, in which the integrated device is formed using semiconductor growth processes, which may include growth processes using low hydrogen content precursors.

The photovoltaic device 100 a depicted in FIG. 1 may be composed of a material stack 50 including a LED device 15 and a photovoltaic device 10 that is present on a supporting substrate 5. The LED device 15 may be present at a first end of the material stack having a first set of bandgap materials 12, 13, 14; and the photovoltaic device at a second end of the material stack having a second set of bandgap materials 8, 9. A band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. It is the energy to promote a valence electron bound to an atom to become a conduction electron, which is free to move within the crystal lattice and serve as a charge carrier to conduct electric current.

In some embodiments, because the LED device 15 and the photovoltaic device 10 are included within the same material stack 50, the devices 10, 15 are monolithographically integrated onto the same substrate 5. In some embodiments, the LED device 15 and the photovoltaic device 10 are in direct contact, i.e., there is no material layer present between and separating the LED device 15 from the photovoltaic device 10. In each of the combinations of electrical devices described herein, the material layers for the photovoltaic device 10 are selected to produce enough power to cause the LED 15 to emit light. In the embodiments in which the photovoltaic device 10 is present atop the LED 15, and the LED 15 emits light in the direction through the photovoltaic device 10, the bandgap materials of the photovoltaic device 10 and the LED device 15 are selected so that the wavelengths of light emitted by the LED 15 are not absorbed by the photovoltaic device 10.

In each of the electrical devices described herein, including a photovoltaic device 10 and a LED device 15 integrated within the same material stack 50 of semiconductor materials, the bandgaps of the materials are selected so that the device composed of the wider bandgap materials, e.g., LED device 15 or photovoltaic device 10, is present at the top of the device, i.e., the light receiving end of the device; and that the device composed of the narrower bandgap materials, e.g., LED device 15 or the photovoltaic device 10, is present at the base of the device, i.e., the end of the device that is furthest from the light source.

Referring to FIG. 1, the photovoltaic device 10 is positioned at the end of the material stack 50 at which at the light (identified by reference number 3) first enters the device, which is the light receiving end of the device. In the embodiment depicted in FIG. 1, the photovoltaic device 10 is present at the light receiving end of the material stack 50 and is composed of a p-type conductivity gallium and nitride containing layer 8, i.e., p-type gallium nitride (GaN), that forms a junction by direct contact to an n-type conductivity gallium and nitride containing layer 9, i.e., n-type gallium nitride (GaN). The band gap for gallium nitride (GaN) is on the order of 3.4 eV. The photovoltaic device 10 that is depicted in FIG. 1 when receiving a light wavelength ranging from 250 nm to 350 nm can provide a voltage that is greater than 2.0 eV. In yet other examples, the voltage produced by the photovoltaic device is greater than 2.25 eV. For example, the photovoltaic device 10 that includes the junction of the n-type gallium nitride (GaN) layer and the p-type gallium nitride (GaN) layer that is depicted in FIG. 1 may produce a voltage of 2.5 V or greater. It is noted that the above examples are provided for illustrative purposes only, and are not intended to limit the present disclosure. In other examples, the voltage produced by the photovoltaic device 10 composed of the n-type and p-type conductivity gallium nitride (GaN) layers depicted in FIG. 1 may be equal to 2.0 V, 2.25 V, 2.5 V, 2.75 V, 3.0 V, 3.25 V, and 3.5 V, as well as any value between the aforementioned examples, and any range of voltages having a lower limit provided by one of the aforementioned example voltages, and an upper limit provided by one of the aforementioned example voltages.

Each of the p-type conductivity gallium and nitride containing layer 8, e.g., p-type gallium nitride (GaN), and the n-type conductivity gallium and nitride containing layer 9, e.g., n-type gallium nitride (GaN) can have a thickness ranging from 100 nm to 2000 nm.

As used herein, “p-type” refers to the addition of impurities to an intrinsic semiconductor that creates deficiencies of valence electrons. As used herein, “n-type” refers to the addition of impurities that contributes free electrons to an intrinsic semiconductor. In a type III-V semiconductor material, the effect of the dopant atom, i.e., whether it is a p-type or n-type dopant, depends upon the site occupied by the dopant atom on the lattice of the base material. In a III-V semiconductor material, atoms from group II act as acceptors, i.e., p-type, when occupying the site of a group III atom, while atoms in group VI act as donors, i.e., n-type, when they replace atoms from group V. Dopant atoms from group IV, such a silicon (Si), have the property that they can act as acceptors or donor depending on whether they occupy the site of group III or group V atoms respectively. Such impurities are known as amphoteric impurities. The dopant that provides the n-type conductivity for the n-type conductivity gallium and nitride containing layer 9 may be present in a concentration ranging from 10¹⁷ atoms/cm³ to 10²⁰ atoms/cm³. The dopant that provides the p-type conductivity of the p-type conductivity gallium and nitride containing layer 8 may be present in a concentration ranging from 10¹⁷ atoms/cm³ to 10²⁰ atoms/cm³.

In some embodiments, the light to power the photovoltaic device 10 of the p-type conductivity gallium and nitride containing layer 8 and the n-type conductivity gallium and nitride containing layer 9 may range from 250 nm to 350 nm. In another embodiment, the light to power the photovoltaic device 10 having the junction of the p-type conductivity gallium and nitride containing layer 8 and the n-type conductivity gallium and nitride containing layer 9 is equal to approximately 300 nm.

The p-type conductivity gallium and nitride containing layer 8 and the n-type conductivity gallium and nitride containing layer 9 of the photovoltaic device 10 are a first set of bandgap materials, while the material layers in the LED device 15 are a second set of bandgap materials. The minimum bandgap for the first set of bandgap materials, i.e., the minimum bandgap materials for the material layers in the photovoltaic device 10, is wider, i.e., greater, than the minimum bandgap for the second set of bandgap materials, i.e., the minimum bandgap materials for the material layers in the LED device 15.

In the embodiment that is depicted in FIG. 1, the end of the material stack 50 at which the light emitting diode (LED) 15 is positioned is further from the light receiving end of the device than the end of the material stack 50 at which the photovoltaic device 10 is positioned. In the embodiment that is depicted in FIG. 1, the LED 15 is positioned between the supporting substrate 4 and the photovoltaic device 10. The supporting substrate 4 can be composed of n-type gallium nitride. The n-type gallium nitride containing photovoltaic junction layer 9 can be in direct contact with the LED device 15. The LED device 15 may include a p-type gallium nitride containing layer 12 that is in direct contact with a first end of a multi quantum well 13, and an n-type gallium nitride containing layer 14 that is direct contact with an opposing second end of the multi quantum well 13. The p-type gallium nitride containing layer 12 of the LED device 15 may be referred to as a first cladding layer, and the n-type gallium nitride containing layer 14 may be referred to as a second cladding layer.

In some embodiments, the first and second cladding layers, i.e., the p-type gallium nitride containing layer 12 and the n-type gallium nitride containing layer 14 of the LED 15, function to pump charge carriers, i.e., electron and hole charge carriers, into the intrinsic active area provided by the quantum well 13 of the LED 15. The dopant that provides the conductivity type, i.e., whether the gallium nitride containing layer 12, 14 is n-type or p-type, may be present in a concentration ranging from 10¹⁷ atoms/cm³ to 10²⁰ atoms/cm³. In some examples, the p-type gallium nitride containing layer 12 and the n-type gallium nitride containing layer 14 may have a thickness ranging from 100 nm to 2000 nm. It is noted that the above compositions and thicknesses are provided for illustrative purposes only, and are not intended to limit the present disclosure.

The active region of the LED device 15 is in the intrinsic (I) region, which is within the quantum well 13. By “intrinsic” it is meant that the region is not doped with an extrinsic dopant, e.g., n-type or p-type dopant, such as the dopants used to dope the first and second cladding layers, i.e., the p-type gallium nitride containing layer 12 and the n-type gallium nitride containing layer 14. The active region in the quantum well structure is formed by alternating layers of relatively low bandgap material and layers of relatively high bandgap material. As used herein, a “low bandgap” is a bandgap ranges from 0.5 eV to 3.0 eV, and a “high bandgap” ranges from 3.1 eV to 3.5 eV. The former layers are termed “well layers” and the latter layers are termed “barrier layers.” For example, for the quantum well 13, e.g., multi-quantum well, the material layers providing the relatively high bandgap material may be gallium nitride (GaN), and the material layers having the relatively low bandgap material may be indium gallium nitride (InGaN). Indium gallium nitride (InGaN) has a band gap of approximately 2.7 eV, while gallium nitride (GaN) has a band gap of approximately 3.4 eV. The band gap of approximately 2.7 eV for the indium gallium nitride (InGaN) portion of the multi-quantum well 13 can provide the minimum value for the second set of bandgap materials for the LED 15 that is less than the minimum value, i.e., is narrower than, the minimum bandgap of the wider bandgap materials for the overlying first set of bandgap materials for the material layers of the photovoltaic device 10.

To provide the stacked structure of quantum wells, the thickness of each layer of semiconductor material within the quantum well may be no greater than 50 nm. For example, the thickness for each layer of the III-V compound semiconductor material, e.g., high band gap GaN and/or low band gap InGaN, within the quantum well 13 may range from 5 nm to 10 nm. In some embodiments, the stacked structure of quantum wells may be composed of 1 to 100 layers of semiconductor material, such as III-V compound semiconductor materials, e.g., the high band gap GaN and/or low band gap InGaN. In yet another embodiment, the stacked structure of quantum wells 13 may be composed of 1 to 5 layers of semiconductor material layers.

The active region consisting of one or more indium gallium nitride (InGaN) including quantum wells 13 sandwiched between thicker layers of gallium nitride (GaN), i.e., cladding layers 12, 14, may provide an LED 15 that provides for blue light emission. For example, the LED device 15 that is depicted in FIG. 1 may emit light having a wavelength of approximately 450 nm. The wavelength of light being emitted by the LED 15 is not absorbed by the photovoltaic device 10 that the light from the LED 15 is being passed through. In some embodiments, by varying the relative In/Ga fraction in the InGaN including quantum wells 13, the light emission can in theory be varied from violet to amber. It is noted that the wavelength of approximately 450 nm is only one example of a light wavelength that may be emitted by the LED 15. In some other examples, the light wavelength that may be emitted by an LED 15 similar to that depicted in FIG. 1 may be equal to 400 nm, 410 nm, 420 nm, 430 nm, 440 nm, 450 nm, 460 nm, 470 nm, 480 nm, 490 nm and 500 nm, as well as any range of values having a lower limit selected from the aforementioned examples, and an upper limit selected from the aforementioned examples, e.g., a range extending from 430 nm to 470 nm. As noted above, the LED 15 may be powered by the voltage produced by the photovoltaic device 10, e.g., a voltage of greater than 2.0 V. In some embodiments, the LED 15 may be powered by the voltage produced by the photovoltaic device 10, e.g., a voltage greater than 2.5 V.

The LED 15 may be positioned on semiconductor substrate 5. In some embodiments, the semiconductor substrate 5 is composed of an n-type III-V semiconductor material, such as gallium nitride, e.g., n-type GaN.

The high voltage GaN photovoltaic cells that are integrated with InGaN LED structures that are depicted in FIG. 1 may also include a glass substrate 4 on the light receiving end of the device.

Contacts 21, 22, 31, 32 may be formed to each of the photovoltaic device 10 and the LED 15. For example, a first LED device contact 21 may be in electrical communication with the n-type gallium nitride (GaN) layer that provides a cladding layer of the LED 15 on a first side of the quantum well 13 of the device, and a second LED device contact 22 may be in electrical communication with the p-type gallium nitride (GaN) layer on the opposing side of the quantum well 13 of the device. A first photovoltaic device contact 31 may be formed on the n-type gallium nitride (GaN) layer 9 of the photovoltaic device 10; and a second photovoltaic device contact 32 may be formed on the p-type gallium nitride layer (GaN) layer of the photovoltaic device 10. Each of the contacts 21, 22, 31, 32 may be composed of an electrically conductive material, such as a metal, e.g., copper, tungsten, aluminum, tantalum, silver, platinum, gold and combinations and alloys thereof.

FIG. 2 depicts another embodiment of an electrical device 100 b including a high voltage photovoltaic device 10 of gallium nitride (GaN) material layers that is integrated with a light emitting diode (LED) 15, in which the photovoltaic device 10 is positioned between the supporting substrate 5 and the LED 15. The high voltage photovoltaic device 10 includes an n-type gallium nitride containing photovoltaic junction layer 9, and a p-type gallium nitride containing photovoltaic junction layer 8. The n-type gallium nitride containing photovoltaic junction layer 9, and a p-type gallium nitride containing photovoltaic junction layer 8 that are depicted in FIG. 2 are similar to the n-type gallium nitride containing photovoltaic junction layer 9, and the p-type gallium nitride containing photovoltaic junction layer 8 that are depicted in FIG. 1. Therefore, the above description of the n-type gallium nitride containing photovoltaic junction layer 9, and the p-type gallium nitride containing photovoltaic junction layer 8 that are depicted in FIG. 1 is suitable for describing the n-type gallium nitride containing photovoltaic junction layer 9, and a p-type gallium nitride containing photovoltaic junction layer 8 that are depicted in FIG. 2.

In the embodiment that is depicted in FIG. 2, the photovoltaic device 10 is present atop the supporting substrate, and the LED 15 is present atop the photovoltaic device 10, wherein the LED 15 is at the end of the material stack 50 that is the light receiving end of the device. In the embodiment depicted in FIG. 2, the photovoltaic device 10 is present between the supporting substrate 5 and the LED device 15.

The LED device 15 includes a p-type gallium nitride containing layer 12, a quantum well 13 composed of gallium nitride containing layers and indium gallium nitride containing layers, and an n-type gallium nitride containing layer 14. The p-type gallium nitride containing layer 12 and the n-type gallium nitride containing layer 14 may be referred to as cladding layers, and can function to pump charge carriers, i.e., electron and hole charge carriers, into the intrinsic active area provided by the quantum well 13. The p-type gallium nitride containing layer 12, the quantum well 13, and the n-type gallium nitride containing layer 14 that are depicted in FIG. 2 are similar to the p-type gallium nitride containing layer 12, the quantum well 13, and the n-type gallium nitride containing layer 14 that are depicted in FIG. 1. Therefore, the above description of the p-type gallium nitride containing layer 12, the quantum well 13, and the n-type gallium nitride containing layer 14 for the LED device 15 that is depicted in FIG. 1 is suitable for describing the p-type gallium nitride containing layer 12, the quantum well 13, and the n-type gallium nitride containing layer 14 of the LED device 15 that is depicted in FIG. 2.

Similar to the electrical device 100 a including high voltage GaN photovoltaic device 10 integrated with an InGaN LED device 15 that is depicted in FIG. 1, the electrical device 100 b including the high voltage GaN photovoltaic device 10 integrated with the InGaN including LED 15 that is depicted in FIG. 2 includes a photovoltaic device 10 that produces power greater than 2.0 V, e.g., 2.5 V or greater power, that is suitable for powering the LED 15. The photovoltaic device 10 may produce the aforementioned levels of power by being subjected to a light source 3 on the order of 300 nm. In response to the voltage, i.e., power, produced by the photovoltaic device 10, the electrically connected LED 15 can emit light having a wavelength on the order of 400 nm or greater, e.g., on the order of approximately 450 nm.

The electrical device 100 b depicted in FIG. 2 also includes a supporting substrate 5, a glass substrate 4, and contacts 21, 22, 31, 32. These structures have been described above by the description of the structures having same reference numbers that are depicted in FIG. 1.

FIG. 3 depicts one embodiment of a high voltage photovoltaic device 10 a including material layers of aluminum gallium nitride (AlGaN) that is integrated with a light emitting diode (LED) device 15 a, in which the LED device 15 a is positioned between the supporting substrate 5 and the photovoltaic device 10. The LED device 15 a may be present at a first end of the material stack 50 a having a first set of bandgap materials 12 a, 13 a, 14 a; and the photovoltaic device at a second end of the material stack having a second set of bandgap materials 8 a, 9 a. In some embodiments, because the LED device 15 a and the photovoltaic device 10 a are included within the same material stack 50 a, the devices 10 a, 15 a are monolithographically integrated onto the same supporting substrate 5. In some embodiments, the LED device 15 a and the photovoltaic device 10 a that are depicted in FIG. 3 are in direct contact, i.e., there is no material layer present between and separating the LED device 15 a from the photovoltaic device 10 a. In each of the combinations of electrical devices described herein, the material layers for the photovoltaic device 10 a are selected to produce enough power to cause the LED 15 a to emit light. In the embodiments, in which the photovoltaic device 10 a is present atop the LED 15 a, and the LED 15 a emits light in the direction through the photovoltaic device 10 a, the bandgap materials of the photovoltaic device 10 a and the LED 15 a are selected so that the wavelengths of light emitted by the LED 15 a are not absorbed by the photovoltaic device 10 a.

In each of the electrical devices described herein, in which the photovoltaic device 10 a and the LED device 15 a are integrated within the same material stack 50 a of semiconductor materials, the bandgaps of the materials are selected so that the device having the wider bandgap materials, e.g., LED 15 a or photovoltaic device 10 a, is present at the top of the device, i.e., the light receiving end of the device; and that the device having the narrower bandgap materials, e.g., LED 15 a or the photovoltaic device 10 a, is present at the base of the device, i.e., the end of the device that is furthest from the light source.

In the embodiment depicted in FIG. 3, the photovoltaic device 10 a is present at the end of the material stack 50 a that is closest to the light source 3 and is composed of a p-type conductivity aluminum, gallium and nitride containing layer 8 a, i.e., p-type aluminum gallium nitride (AlGaN), that forms a junction by direct contact to an n-type conductivity aluminum, gallium and nitride containing layer 9 a, i.e., n-type aluminum gallium nitride (AlGaN). The band gap for aluminum gallium nitride (AlGaN) is on the order of 4 eV. The photovoltaic device 10 a that is depicted in FIG. 3 when receiving a light wavelength ranging from 200 nm to 300 nm can provide a voltage that is greater than 2.5 V. In yet other examples, the voltage provided is greater than 3.0 V. For example, the photovoltaic device 10 a that is depicted in FIG. 3 that is composed of a junction of n-type and p-type aluminum gallium nitride (AlGaN) layers may provide a voltage of 3.5 V or greater. It is noted that the prior examples are provided for illustrative purposes only, and are not intended to limit the present disclosure. In other examples, the voltage provided by a photovoltaic device 10 a composed of a junction of n-type and p-type conductivity aluminum gallium nitride (AlGaN), as depicted in FIG. 3, may be equal to 2.5 V, 2.75 V, 3.0 V, 3.25 V and 3.5V, as well as any value between the aforementioned examples, and any range of voltages having a lower limit provided by one of the aforementioned examples, and an upper limit provided by one of the aforementioned examples.

Each of the p-type conductivity aluminum, gallium and nitrogen containing layer 8 a, i.e., p-type aluminum, gallium nitride (AlGaN), and the n-type conductivity aluminum, gallium and nitrogen containing layer 9 a, i.e., n-type aluminum gallium nitride (AlGaN) may have a thickness ranging from 100 nm to 2000 nm. The dopant that provides the p-type conductivity of the n-type conductivity aluminum, gallium and nitride containing layer 8 a may be present in a concentration ranging from 10¹⁷ atoms/cm³ to 10²⁰ atoms/cm³. The dopant that provides the n-type conductivity of the n-type conductivity aluminum, gallium and nitride containing layer 9 a may be present in a concentration ranging from 10¹⁷ atoms/cm³ to 10²⁰ atoms/cm³.

In some embodiments, the light to power the photovoltaic device 10 a of the p-type conductivity aluminum, gallium and nitride containing layer 8 a and the n-type conductivity aluminum, gallium and nitride containing layer 9 a may range from 200 nm to 325 nm. For example, the light to power the photovoltaic device 10 a having the junction of p-type AlGaN and n-type AlGaN is equal to approximately 250 nm.

The p-type conductivity aluminum, gallium and nitride containing layer 8 a and the n-type conductivity aluminum, gallium and nitride containing layer 9 a of the photovoltaic device 10 a provide a first set of bandgap materials, while the material layers in the LED 15 a provide a second set of bandgap materials. The minimum bandgap for the first set of bandgap materials, i.e., the minimum bandgap materials for the material layers in the photovoltaic device 10 a, is wider than the minimum bandgap for the second set of bandgap materials, i.e., the minimum bandgap materials for the material layers in the LED 15 a.

Referring to FIG. 3, the second end of the material stack 50 a includes the light emitting diode (LED) 15 a. In the embodiment that is depicted in FIG. 3, the LED 15 a is positioned between the supporting substrate 5 and the photovoltaic device 10 a, wherein the supporting substrate 5 is comprised of n-type aluminum gallium nitride (n-type AlGaN). The n-type aluminum gallium nitride containing photovoltaic junction layer 9 a is in direct contact with the LED device 15 a. The LED device 15 a may include a p-type aluminum gallium nitride containing layer 12 a that is in direct contact with a first end of a multi quantum well 13 a, and an n-type aluminum gallium nitride containing layer 14 a that is direct contact with an opposing second end of the multi quantum well 13 a. The p-type aluminum gallium nitride containing layer 12 a of the LED 15 a may be referred to as a first cladding layer, and the n-type gallium nitride containing layer 14 a may be referred to as a second cladding layer.

In some embodiments, the first and second cladding layers, i.e., the p-type aluminum gallium nitride containing layer 12 a and the n-type aluminum gallium nitride containing layer 14 a, function to pump charge carriers, i.e., electron and hole charge carriers, into the intrinsic active area provided by the quantum well 13 a. The dopant that provides the conductivity type, i.e., whether the aluminum gallium nitride containing layer 12 a, 14 a is n-type or p-type, may be present in a concentration ranging from 10¹⁷ atoms/cm³ to 10²⁰ atoms/cm³. In some examples, the p-type aluminum gallium nitride containing layer 12 a and the n-type aluminum gallium nitride containing layer 14 a may have a thickness ranging from 100 nm to 2000 nm. It is noted that the above compositions and thicknesses are provided for illustrative purposes only, and are not intended to limit the present disclosure.

The active region of the LED 15 a is in the intrinsic (I) region, which is present in the quantum well 13 a. The active region in the quantum well structure is formed by alternating layers of relatively low bandgap material and layers of relatively high bandgap material. As used herein, a “low bandgap” is a bandgap ranges from 0.5 eV to 3.75 eV, and a “high bandgap” ranges from 3.8 eV to 4.5 eV. The former layers are termed “well layers” and the latter layers are termed “barrier layers.” For example, for the quantum well 13 a, e.g., multi-quantum well, the material layers providing the relatively high bandgap material may be aluminum gallium nitride (AlGaN) having a bandgap of approximately 4 eV, and the material layers having the relatively low bandgap material may be gallium nitride (GaN) having a bandgap of approximately 3.4 eV. The band gap of approximately 3.4 eV for the gallium nitride (GaN) portion of the multi-quantum well provides that the minimum value for the second set of bandgap materials for the material layers in the LED 15 a is less than the minimum value, i.e., is narrower, than the minimum bandgap of the wider bandgap materials for the overlying first set of bandgap materials for the material layers in the photovoltaic device 10 provided by aluminum gallium nitride (AlGaN) having a band gap of 4.0 eV.

To provide the stacked structure of quantum wells, the thickness of each layer of semiconductor material within the quantum well may be no greater than 50 nm. For example, the thickness for each layer of the III-V compound semiconductor material, e.g., high band gap AlGaN and/or low band gap GaN, within the quantum well 13 a may range from 5 nm to 10 nm. In some embodiments, the stacked structure of quantum wells may be composed of 1 to 100 layers of semiconductor material, such as III-V compound semiconductor materials, e.g., the high band gap AlGaN and/or low band gap GaN. In yet another embodiment, the stacked structure of quantum wells may be composed of 1 to 5 layers of semiconductor material layers.

The active region consisting of one or more gallium nitride (GaN) and aluminum gallium nitride (AlGaN) including quantum wells 13 a sandwiched between thicker layers of n-type and p-type doped aluminum gallium nitride (AlGaN) layers 12 a, 14 a, i.e., cladding layers 12 a, 14 a, may provide an LED 15 a that provides for ultra violet (UV) light emission, i.e., light emission at wavelengths of 400 nm or less. For example, the LED device 15 a that is depicted in FIG. 3 may emit light having a wavelength of approximately 350 nm. The wavelength of light being emitted by the LED device 15 a is not absorbed by the photovoltaic device 10 a that the light from the LED device 15 a is being passed through. It is noted that the wavelength of approximately 350 nm is only one example of a light wavelength that may be emitted by the LED device 15 a. In some other examples, the light wavelength that may be emitted by an LED device 15 a similar to that depicted in FIG. 3 may be equal to 300 nm, 310 nm, 320 nm, 330 nm, 340 nm, 350 nm, 360 nm, 370 nm, 380 nm, 390 nm and 400 nm, as well as any range of values having a lower limit selected from the aforementioned examples, and an upper limit selected from the aforementioned examples, e.g., a range extending from 330 nm to 370 nm. As noted above, the LED device 15 a may be powered by the voltage produced by the photovoltaic device 10 a, e.g., a voltage of greater than 2.5 V. In some embodiments, the LED device 15 a may be powered by the voltage produced by the photovoltaic device 10 a, e.g., a voltage greater than 3.0 V.

The LED device 15 a may be positioned on semiconductor substrate 5. In some embodiments, the semiconductor substrate 5 is composed of an n-type III-V semiconductor material, such as aluminum gallium nitride, e.g., n-type AlGaN.

The high voltage GaN photovoltaic cells that are integrated with InGaN LED structures that are depicted in FIG. 3 may also include a glass substrate 4 on the light 3 receiving end of the device.

The electrical device 100 c depicted in FIG. 3 also includes contacts 21, 22, 31, 32. These structures have been described above by the description of the structures having same reference numbers that are depicted in FIG. 1.

FIG. 4 depicts another embodiment of an electrical device 100 d including a high voltage photovoltaic device 10 b of gallium nitride (GaN) material layers that is integrated with a light emitting diode (LED) 15, in which the photovoltaic device 10 b is positioned between the supporting substrate 5 and the LED 15 b. The high voltage photovoltaic device 10 b includes an n-type gallium nitride containing photovoltaic junction layer 9 b, and a p-type gallium nitride containing photovoltaic junction layer 8 b. The n-type gallium nitride containing photovoltaic junction layer 9 b, and a p-type gallium nitride containing photovoltaic junction layer 8 b that are depicted in FIG. 4 are similar to the type gallium nitride containing photovoltaic junction layer 9, and a p-type gallium nitride containing photovoltaic junction layer 8 that are depicted in FIG. 1. Therefore, the above description of the n-type gallium nitride containing photovoltaic junction layer 9, and the p-type gallium nitride containing photovoltaic junction layer 8 that are depicted in FIG. 1 is suitable for describing the n-type gallium nitride containing photovoltaic junction layer 9 b, and a p-type gallium nitride containing photovoltaic junction layer 8 b that are depicted in FIG. 4.

In the embodiment that is depicted in FIG. 4, the LED 15 b is present atop the supporting substrate 5, and the photovoltaic device 10 b is present atop the LED 15 b, wherein the LED 15 b is at the end of the material stack 50 that is the light receiving end of the device. In the embodiment depicted in FIG. 4, the LED 15 b is present between the supporting substrate 5 and the photovoltaic device 10 b.

The LED 15 b includes a p-type aluminum gallium nitride containing layer 12 b, a quantum well 13 b composed of aluminum gallium nitride containing layers and indium gallium nitride containing layers, and a n-type aluminum gallium nitride containing layer 14 b.

The p-type aluminum gallium nitride containing layer 12 b and the n-type aluminum gallium nitride containing layer 14 b may be referred to as cladding layers, and can function to pump charge carriers, i.e., electron and hole charge carriers, into the intrinsic active area provided by the quantum well 13 b. The p-type aluminum gallium nitride containing layer 12 and the n-type aluminum gallium nitride containing layer 14 that are depicted in FIG. 4 are similar to the p-type aluminum gallium nitride containing layer 12 a, and the n-type aluminum gallium nitride containing layer 14 a that are depicted in FIG. 3. Therefore, the above description of the p-type aluminum gallium nitride containing layer 12 a and the n-type aluminum gallium nitride containing layer 14 a for the LED device 15 a that is depicted in FIG. 3 is suitable for describing the p-type aluminum gallium nitride containing layer 12 b and the n-type aluminum gallium nitride containing layer 14 b of the LED device 15 b that is depicted in FIG. 4.

The active region of the LED device 15 b is in the intrinsic (I) region, which is within the quantum well 13 b. The active region in the quantum well structure is formed by alternating layers of relatively low bandgap material and layers of relatively high bandgap material. As used herein, a “low bandgap” is a bandgap ranges from 0.5 eV to 3.4 eV, and a “high bandgap” ranges from 3.5 eV to 4.5 eV. The former layers are termed “well layers” and the latter layers are termed “barrier layers.” For example, for the quantum well 13 b, e.g., multi-quantum well, the material layers providing the relatively high bandgap material may be aluminum gallium nitride (AlGaN), and the material layers having the relatively low bandgap material may be indium gallium nitride (InGaN). Indium gallium nitride (InGaN) has a band gap of approximately 2.7 eV, while aluminum gallium nitride (GaN) has a band gap of approximately 4.0 eV.

To provide the stacked structure of quantum wells, the thickness of each layer of semiconductor material within the quantum well may be no greater than 50 nm. In some embodiments, the stacked structure of quantum wells may be composed of 1 to 100 layers of semiconductor material. In yet another embodiment, the stacked structure of quantum wells 13 b may be composed of 1 to 5 layers of semiconductor material layers.

The electrical device 100 d depicted in FIG. 4 also includes a supporting substrate 5, a glass substrate 4, and contacts 21, 22, 31, 32. These structures have been described above by the description of the structures having same reference numbers that are depicted in FIG. 1.

The structures depicted in FIGS. 1-4 are now described with more detail in the following description of methods for forming high voltage photovoltaic cells that are integrated with LEDs.

FIG. 5 is a flow chart illustrating one embodiment of a method for forming high voltage photovoltaics that are integrated with LEDs, such as those depicted in FIGS. 1 and 3, in which the method includes a low hydrogen deposition process. The term “low hydrogen” denotes that the deposition step has a maximum hydrogen content of 1×10¹⁸ cm⁻³.

The method may begin at step 101 with forming lower junction by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). The lower junction may be formed on a supporting substrate 5, such as an n-type gallium nitride (GaN) substrate, as depicted in FIGS. 1 and 3. The lower junction that is formed may provide an LED 15, 15 a, as depicted in FIGS. 1 and 3. For example, the lower junction may provide an LED 15 depicted in FIG. 1 is composed of a p-type gallium nitride layer 12, as a first cladding layer of the LED 15; a quantum well of including gallium nitride and indium gallium nitride layers; and an n-type gallium nitride layer 14, as a second cladding layer of the LED 15. For example, the lower junction may provide an LED 15 a depicted in FIG. 3 is composed of a p-type aluminum gallium nitride layer 12 a, as a first cladding layer of the LED 15 a; a quantum well 13 a including gallium nitride and aluminum gallium nitride layers; and an n-type aluminum gallium nitride layer 14 a, as a second cladding layer of the LED 15 a.

The material layers of the lower junction may be formed using epitaxial growth. The terms “epitaxial growth and/or deposition” means the growth of a semiconductor material on a deposition surface of a semiconductor material, in which the semiconductor material being grown has substantially the same crystalline characteristics as the semiconductor material of the deposition surface. The term “epitaxial material” denotes a material that is formed using epitaxial growth. In some embodiments, when the chemical reactants are controlled and the system parameters set correctly, the depositing atoms arrive at the deposition surface with sufficient energy to move around on the surface and orient themselves to the crystal arrangement of the atoms of the deposition surface. Thus, in some examples, an epitaxial film deposited on a {100} crystal surface will take on a {100} orientation.

The epitaxial growth process may be by chemical vapor deposition (CVD) or molecular beam epitaxy (MBE) growth processes.

MBE growth processes can include heat the substrate, which can be to some hundreds of degrees (for example, 500-600° C.) in the case of gallium nitride). In a following step, MBE growth processes include a precise beam of atoms or molecules (heated up so they're in gas form) being fired at the substrate from “guns” called effusion cells. The composition of the molecules being fired in the beams provide the composition of the deposited material layer. The molecules land on the surface of the substrate, condense, and build up systematically in ultra-thin layers, so that the material layer being grown forms one atomic layer at a time.

Chemical vapor deposition (CVD) is a deposition process in which a deposited species is formed as a result of chemical reaction between gaseous reactants at greater than room temperature (25° C. to 900° C.); wherein solid product of the reaction is deposited on the surface on which a film, coating, or layer of the solid product is to be formed. Variations of CVD processes include, but not limited to, Atmospheric Pressure CVD (APCVD), Low Pressure CVD (LPCVD) and Plasma Enhanced CVD (PECVD), Metal-Organic CVD (MOCVD) and combinations thereof may also be employed. In some preferred embodiments, the CVD process used to form the lower junction may be metal organic chemical vapor deposition.

A number of different sources may be used for the deposition of epitaxial type III-V semiconductor material. In some embodiments, the sources for epitaxial growth of type III-V semiconductor material include solid sources containing In, Ga, N, P elements and combinations thereof and/or a gas precursor selected from the group consisting of trimethylgallium (TMG), trimethylindium (TMI), Trimethylaluminum (TMA), tertiary-butylphosphine (TBP), phosphine (PH₃), ammonia (NH₃), and combinations thereof. The temperature for epitaxial deposition of type III-V semiconductor materials may range from 550° C. to 900° C. Although higher temperature results in faster deposition, the faster deposition may result in crystal defects and film cracking.

The material layers for the lower junction may be doped n-type or p-type using in situ doping. By “in-situ” it is meant that the dopant that provides the conductivity type of the material layer, e.g., material layer that contributes to providing a junction, is introduced as the material layer is being formed. To provide for in-situ doped p-type or n-type conductivity, the dopant gas may be selected from the group consisting of bis-cyclopentadienyl-magnesium (Cp₂Mg), silane (SiH₄), disilane (Si₂H₆), germane (GeH₄), carbon tetrabromide (CBr₄) and combinations thereof. The intrinsic materials of the quantum wells 13, 13 a are not doped with n-type or p-type dopant.

In a following step 102, the dopants within the first junction are activated. Activation anneal may be conducted at a temperature ranging from 850° C. to 1350° C. Activation annealing may be provided by furnace annealing, rapid thermal annealing (RTA) or laser annealing.

At step 103, the method continues by forming the upper junction using a low hydrogen deposition process, such as MBE growth methods. In the examples illustrated in FIGS. 1 and 3, the upper junction is a photovoltaic device 10, 10 a that is epitaxially formed directly on LED 15, 15 a. In the embodiment depicted in FIG. 1, the photovoltaic device 10 may include a p-type gallium nitride (p-type GaN) layer 8 and an n-type gallium nitride (n-type GaN) layer 9. In the embodiment depicted in FIG. 3, the photovoltaic device 10 a may include a p-type aluminum gallium nitride (p-type AlGaN) layer 8 a and an n-type aluminum gallium nitride (n-type AlGaN) layer 9 a.

It has been determined that hydrogen precursors can deactivate the electrically activated p-type dopant in the underlying p-type gallium nitride containing layers and/or p-type aluminum gallium containing nitride layers of the underlying LEDs 15, 15 a. Therefore, the method for depositing the material layers of the photovoltaic devices 10 a, 10 b can employ a low-hydrogen containing process, e.g., deposition method using hydrogen free precursors, such as MBE. A second activation anneal may be formed after the formation of the upper junction. The activation anneal may be conducted at a temperature ranging from 850° C. to 1350° C. Activation annealing may be provided by furnace annealing, rapid thermal annealing (RTA) or laser annealing.

At step 104, the upper and lower junction may be patterned and etched to provide the geometry for each photovoltaic device 10, 10 a, 10 b, and each LED 15, 15 a, 15 b. The upper and lower junctions may be patterned using photolithography and etch processes. For example, a photoresist mask may be formed on the uppermost semiconductor layer by applying a photoresist layer, exposing the photoresist layer to a pattern of radiation, and then developing the pattern into the photoresist layer utilizing conventional resist developer. In some embodiments, the block masks have a thickness ranging from 100 nm to 300 nm. The exposed potions of the semiconductor material layers that provide the upper and lower junctions may then be etched using an etch process, such as an anisotropic etch, e.g., reactive ion etch (RIE), or an isotropic etch, such as a wet chemical etch. In some embodiments, a first pattern and etch sequence including a first etch mask may be used to define the geometry of the upper junction; and a second pattern and etch sequence including a second etch mask may be used to define the geometry of the lower junction.

Thereafter, the contacts 21, 22, 31, 32 may be formed to each of the upper and lower junctions, i.e., photovoltaic device 10, 10 a, 10 b and the LED 15, 15 a, 15 b, using deposition, photolithography and etching processes. For example, a metal layer can be deposited using a physical vapor deposition (PVD) process. The PVD process may include plating, electroplating, electroless plating and combinations thereof. The deposited metal layers may be patterned and etched using deposition, photolithography and etching to provide the desired geometry of the contacts 21, 22, 31, 32.

FIG. 6 is a flow chart of another embodiment of a method for forming high voltage photovoltaics that are integrated with LEDs. The process flow depicted in FIG. 6 does not require a low hydrogen forming method. At steps 201 and 202 the lower and upper junctions may be epitaxially formed. In some embodiments, the lower and upper junctions may be formed using the chemical vapor deposition, e.g., MOCVD, and MBE forming methods that have been described above with reference to FIG. 5 for steps 101 and 103.

At step 203, at least the upper junction of the device is patterned to provide the geometry of the desired photovoltaic device 10, 10 a, 10 b and/or the LED 15, 15 a, 15 b. The pattern step described in step 203 is similar to the patterning step described at step 104 of the process flow describe with reference to FIG. 5. Therefore, the above described photolithography and etch steps described with reference to step 104 of FIG. 5 are applicable to an least one example of the patterning step for step 203 of the process flow illustrated in FIG. 6.

At step 204, the n-type and p-type dopants for the material layer of the upper and lower junction are activated by an activation anneal. The activation anneal at step 204 of the process flow for the method in FIG. 6 is similar to the activation anneal at step 102 of the method illustrated in FIG. 5. Therefore, the above description of the activation anneal for step 102 for the method illustrated in FIG. 5 is suitable for describing one embodiment of the activation anneal that can be used at step 204 of the method depicted in FIG. 6.

At step 205, contacts 21, 22, 31, 32 are formed to the upper and lower junctions of the device.

It is to be appreciated that the use of any of the following “/”, “and/or”, and “at least one of”, for example, in the cases of “A/B”, “A and/or B” and “at least one of A and B”, is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of both options (A and B). As a further example, in the cases of “A, B, and/or C” and “at least one of A, B, and C”, such phrasing is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of the third listed option (C) only, or the selection of the first and the second listed options (A and B) only, or the selection of the first and third listed options (A and C) only, or the selection of the second and third listed options (B and C) only, or the selection of all three options (A and B and C). This may be extended, as readily apparent by one of ordinary skill in this and related arts, for as many items listed.

Having described preferred embodiments of a system and method (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims. 

1. A method of forming an electrical device comprising: growing a first junction on a supporting substrate; forming a second junction on the first junction by molecular beam epitaxial growth; and forming contacts to the first junction and the second junction, wherein a first device of the first and the second junction is a photovoltaic device, and a second device of the first and the second junction is a light emitting diode, wherein the semiconductor material layers in the LED or the photovoltaic device that is at a light receiving end of the electrical device have a wider band gap than the LED or the photovoltaic device that is not at the light receiving end of the electrical device.
 2. The method of claim 1, wherein the molecular beam epitaxial growth is free of hydrogen.
 3. The method of claim 1, wherein said growing the first junction comprises chemical vapor deposition or molecular beam epitaxial growth.
 4. The method of claim 1, wherein forming the contacts comprises: etching the second junction; etching the first junction; and forming metal electrodes to the first and second junction.
 5. The method of claim 1, wherein the photovoltaic device comprises an n-type gallium nitride layer and a p-type gallium nitride layer.
 6. The method of claim 1, wherein the photovoltaic device comprises a p-type aluminum gallium nitride layer and an n-type aluminum gallium nitride layer.
 7. The method of claim 1, further comprising activation annealing of the first junction prior to forming the second junction.
 8. The method of claim 7, wherein said activation annealing comprises a temperature ranging from 850° C. to 1350° C. 